The term "no-mix orthodontic adhesives" is used to refer to two-package adhesive systems that are not mixed prior to applying to the tooth to bond an orthodontic bracket thereto. A typical example is an acrylic resin system consisting of two packages; one part contains an acrylic resin and an accelerator and the other contains an acrylic resin and a catalyst. Typically, a "primer" coating of unfilled resin and, e.g., the accelerator, is applied to the surface of the tooth and to the bonding surface of the bracket, and the other part of the two-package system (a filled resin system containing the catalyst) is then interposed between the two primer coats. The accelerator then activates the catalyst when the two parts of the system contact each other, and the resin is thereby cured. Examples of such no-mix systems are described by Lee et al. in U.S. Pat. No. 4,340,529 and by Johnston in U.S. Pat. Nos. 4,200,980 and 4,363,624.
No-mix adhesives seldom achieve the ultimate bond strength achieved by the more conventional two package chemically cured adhesives that are mixed prior to use. However, no-mix adhesives are gaining favor in the orthodontic profession because the no-mix adhesives are easier to use and do provide certain functional advantages. First, a conventional two-part mix adhesive has a finite working time after it has been mixed, usually of the order of 2-3 minutes. Thus, after such an adhesive has been mixed, the dentist must hurry to use it before it sets up. In contrast, the no-mix adhesive is not "mixed" until the paste portion is placed in contact with the liquid ("primer") portion, just as the bracket is being placed. Therefore, the dentist can work with one bracket at a time at a non-hurried pace. In many cases, with mix adhesives, a significant proportion of the mixed formulation sets up before it is used, with resulting waste of material. Also, once the no-mix adhesive has been applied, the setting time is quite short (e.g., 20-45 seconds); therefore, there is less chance that the bracket will drift out of place once it has been positioned on the tooth than is the case with mix adhesives wherein the setting time is longer.
Since the advent of orthodontic brackets made of clear materials such as polycarbonate, and more recently single crystal alumina or clear sapphire, there has been a need to employ bracket adhesives that resist staining from foods and beverages. Obviously the aesthetic advantage of using a clear bracket would be somewhat diminished if the bracket adhesive were to discolor. This invention provides a no-mix orthodontic bracket adhesive that achieves a bond strength equal to or better than current no-mix adhesives that are commercially available, and which resists staining from beverages and foodstuffs significantly better than any of the currently available commercial no-mix orthodontic adhesives.